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ORIGIN OF PETROLEUM EVOLUTION OF PETROLEUM 1. C. MNab. P. V. Smith, &.,and R. L B*b
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COMPOSITION OF PETROLEUM COMPMlTlON OP U. S. CRUDE OILS Hvdd M.Sdtb
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ORIGIN OF PETROLEUM
The Evolution of Petroleum J. G . MCNAB, P. V. SMITH, JR., AND R. L. BETTS Esso Laboratories, Standard Oil Devel&nent Co., Linden, N. J.
I
N T H E past several decades, very large quantities of petroleum in commercial accumulstions have been discovered, largely by the application of geological and geophyeical methods. Although petroleum is a composition consisting almost entirely of organic chemical compounds, past discoveries have not been facilitated to any measurable degree by the limited knowledge available, from the chemical standpoint, on the origin and mecbanism of formation of petroleum. Many of the basic tenets of petroleum origin have, in fact, been established on the basis of accumulated geological observations derived from exploration activities. These tenets include the organic nature of the mwce material from which petroleum is derived, the marine or brackish water environment of deposition of this Source material for wentially all of the world'e known oil fields, and the inthence of temperature. as a result of deep burial of sediments containing the organic Sonice material, on the transformation of the organic matter into petroleum. With the increasing consumption of petroleum anticipated for some years to come and the attendant importance of maintaining and augmenting proved reserves, the search for oil is being pushed to lower depths and to less accessible and perhaps less promising meas. Exploration difficulties and costs can, therefore, be expected to increase. As a result, an understanding of the origin and mechanism of formation of petroleum is becoming
of greater importance. The degree to which exploration activities could be facilitated or made more effectiveby such an nnderstanding is difficult to appraise. It is to be expected, however, that an accurate picture of the origin and genesis of petroleum would provide diagnostic criteria which could be applied to advantage in locating new petroliferous areaa and developing newly eatablished oil-bearing regions. In this connection it is pertinent to quote from one familiar with exploration and producing problems. I n 1946, Knebel of the Stmdard Oil Co. (N. J.), concluded a review (6) of A.P.I. Project 43 studies with the following statement: "If we know how oil ia formed, we can find better ways and means of loca;tiogthe accumulations." Petroleum is normally found in coarse-grained, porous reservoir rocks, such sandstones and limestones, in which the pore spaces are occupied by the oil. These rocks were formed from inorganic sediments deposited in ancient Seas which, a t various times in past geologic e r a , covered much of the present lend areas where petroleum is now found. Always associated with the porous reservoir or trap rocks in which petroleum is found are finegrained rocks containing appreciable quantities of organic matter. The oil is considered to have been derived from the organic matter deposited with the finegrained sediments which, on subsequent deep burial, were compacted into rocks. The universal association of finegrained rocks Containing organic
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120th. and 121st Meetings of the American Chemical Societ
COMPOSITION OF PETROLEUM (Continued) NITROGEN COMPOUNM IN DOMESTIC HEATING OIL DISTILLATE R W.R.s.ur, F. W. M.&ur,md R A . B m m
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PETROLEUM ASH COMPONENTS AND THEIR EFFECT ON REFRACTORIES MhrC.K.hrmdR*rtLHard,
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CYCLIC SULFIDES IN A PETROLEUM DISTILLATE k...lK L a m u d S w r w M w n r . .
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SHALE OIL COMPOSITION OF AMERICAN AND FOREIGN CRUDE SHALE O I I S l
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matter with petroleum reservok and the widespread presence
deposita, the inorganic constituents are compacted into rocks, and the sensitive organic matter, under the influence of the increased temperatures (probably in tbe mge 160' to mmpounda, and optically active components have established that petroleum is of organic origin. 300' F.), is cracked over an extended period of perhape 1,oM),000 From an extensive study of available information, it is conto 2,000,000 years to a lower molecular weight bitumenlike cluded that the origin and evolution of petroleum embody material or heavy naphthenic oil. Tbe original oils formed are three distinct steps or phases: squeezed out of the compacted he-grained rocks and accumulate 1. Accumulation of organic matter in marine or brackish by migration where favorable structural conditions, such as water nediments porous reservoir rock and impervious cap mck, prevail. The 2. Transformation of the organic matter, or psrt of it, t o a heavy oils first formed probably contain little or no low molecheavy bitumenlike material or embryonic petroleum ular weight (low boiling) hydrocarbons, and, under the iduence of moderate temperature, geologic time f h r , and catalysie by 3. Maturing of the primary petroleum I n the firat step organic residues, consisting of the remnants components of the mmvoir rock undergo progwsive evolution. of marine animal or vegetable life (probably in large measure The reactions involved in the maturing procees are believed to marine plsnkton and bacteris) accumulate with large proportions ' be low temperatun? cracking or hydrocracking in which hydrogen of inorganic sediments in t r a n s f e r from naphthenic favorable marine or brackish rings to unaaturated fragIn order to obtain a better undermtanding of the evoluwater environments. A ments OCCUIE. tion of petroleum, an extensive d e w of aude oil assay minor proportion of the total Much information along data for United State. oil fields has been made. T h e study organic matter accumulating chemical lines is needed to in the sediments is believed to anphasims that chemical changes in the pricrude clarify the picture relating to oils after accumulation in m S e N O i r 8 are probabb. A consist of high molecular the origin and evolution of general increase in the A.P.I. gravity and in the volatility weight, carbon- and bydrcpetroleum. It is believed that of the crude oils is observed with increasing depth of gfQl-rich components formed c o n s i d e r a b l e contributions by bioohemiml reduction recan be made by aiepisoant burial. Similar data were obtained in experimental actions in the life processes chemical studies of limited work on so- Venezuelan and Canadian oil.. Thermal scope, although a complete of the lower f o m of marine aackiig studies on a "young" aude indicated that it solution to the problem will would not be Ugnifiuntly changed at a temperature of oganisms. This type of 1 W F . within geologic time. I t was m o m sensitive to organic matter iu believed undoubtedly require many aacking than the average gas oil, however. and at 250. F. to be sensitive to chemical years of study in a number of change such as c r a c k g or fields of science. it would be 99% converbd to lighter product. in 4OO,OOO,OOO The work covered in the years. Contact of such young oils with mildly catalytic depo?vmerization. As t h e present paper is concerned strata during a portion of their history might permit sediments become d e e p l y with the evolution of petrccomplete convanion at the lower temperat-. buried u'nder s u b s e q u e n t
in crude oils of chlorophyll derivatives, nitrogen-containing
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leuni. I t is generally me11 known that most of the crude oils produced from very old sedimentary rocks are light, paraffinic products containing large proportions of low molecular weight components-that is, hydrocarbons boiling in the gasoline range. I n general, also, the crudes produced from the youngest formations are heavy, naphthenic oils containing often little or no light liquid hydrocarbons. Although the characteristir differences between crude oils found in sedimentary rocks of diffwent ages and a t different producing depths may be due to, and undoubtedly are due in part to differences in the original source materials resulting from different environmental conditions, there is also considerable evidence that the first crude petroleum formed goes through a progrewive fierirs of metamorphic steps in the reservoir rock.
temperature cracking where lightcr fragments are formed and lower boiling componcn t,s are produced. From a thermodynamic standpoint, such a change toward lower rnolecular weight hydrocarbons is sound, the free energy of formation values for all classes of hydrocarbons (paraffins, naphthenes, aromatics) decreasing in general from high positive values in the high molecular weight range to lower positive values (or iicgative values in the paraffins) for the lowest molecular weight members of a series. The changes in properties of the crude oils m-ere studied as functions
EVIDENCE OF EVOLUTION Perhaps the most general indication that evolu tion occurs in petroleurn lies in the fact that the crude oils found in the younger sedimentary formations are the ones that contain appreciable quantities of nitrogen- and oxygen-containing components. This obsrrvation shows a genrtic relationship betaern these younger oils and the original organic source material from which the petroleum was formed. The crude oils found in the older sediments are characteristically low in content of oxygen- and nitrogen-containing materials indicating that a maturing. in the direction of conversion of the remaining vestiges of the original source material, has occurred. That petroleum goes through a progressive series of maturing steps has been postulated as far back as 1915 by White (8) who found a correlation between the rank or dcgrec of maturity of coal deposits and that of oils in regions where these two fossil fuels occurred in associated sediments. The probable evolution of petroleum was also emphasized by Emmons (4)later in 1931 and inore recently by Barton (6) The conclusions of these investigators, however, were based on rather limited available data. SIGNIFICANCE O F CRUDE OIL ASSAY DATA United States Crude Oils. I n the present study a review has been made of extensive crude oil assay data for United States crudes made available in the Bureau of Mines' Report of Investigations 4289 (6). In this publication crude oil assay data were tabulated for 283 out of 340 oil fields in the IJnited States producing more than 2500 barrels per day of crude as of January 1, 1947. A few crudes were excluded from the present etudy because producing formation and age data were lacking. Also excluded were a fen- cases where more than one producing formation and age were indicated for a given field. The sinall number of condensate fields listed was also omitted for present purposes. A summary showing the number of fields included in the review is given in Table I. The results of the study of the crude oil assay data are outlined in the follosing paragraph?. Table I.
Scope of Study on Evolution of Petroleum (6) No. of Crudes Producing formation Producing formation, age, a n d depth given a n d age given
Tertiary Mesozoic Paleozoic Total Total Tertiary Gulf Coast California
112
33
106 -
261 112 76 36
95
28 90
213 9.5
67
28
The following bases were used as evidences of progressive ~volutionin crude oils: First, the A.P.I. gravity of crudes and, secondly, the proportion of low boiling hydrocarbons-that is, components boiling in the gasoline range-were considered as evidence of evolution. These criteria of crude oil evolution are baPed on the probable mechanism involved being one of low 2558
Figure 1
of ago and depth of producing horizons. The significance of producing depth is that tciiipcratures iri sedimentary rocks increase with depth. an average gradient being in the order of an increase in temperature of about 1' F. for 50 feet of burial. The rate of metamorphism of an oil in a reservoir, thcrefore, should be a function of both time and depth. Figure 1 shows tho relationship between the A.P.1. gravities of crude oils and the age of thesr oils I n this figure the percentages of crudes having gravities higher than 30" A.P.I. and lower than 30" A.P.I. are shoxn for crude oils of the three major geologic eras. Thc Tcrtiary crudes are the relatively young oils found in sedimentary rocks with an estimated age of from about 1I,OOO,OOOyears for Pliocene to about 74,000,000 years for Eocene. The Mesozoic era oils cover an estimated time interval of from about 75,000,000 t o 200,000,000 year.. The Paleozoic crudes arc found in the older rocks, and the age of such rocks is roughly froin 200,000,000 to about 500,000,000 years. Figure 1 also shows that the proportion of high gravity crudes-that is, those having an 4.P.I. gravity of above 3Oo-is much greater in the crudes from the older rocks. Sctually the ratio of high gravity to low gravity crudes in thc Tertiary is 62/38 compared to a ratio of 88/12 for the oils produced from Paleozoic formations. A summary of the gravity-age data for the crude oils studied is as follows:
% > 30" A.P.I.
70 < 30' A.P.I.
Tertiary (112 Crudes) 62 38
Mesozoic (33 Crudes) 69 31
Paleozoic (106 Crudes) 88 12
In Figure 2 thr disiributioii of A.P.I. gravities of crude oils in relation to producing depth is shown For the purposes of this study, only the Tertiary crude oils wcre considered. Producing depth figures for oils in older srdimentary rocks are not reliable measures of depth of burial, since in many cases large proportione of the sedimentary rocks originally laid down have been eroded from the top of the sedimentary areas. Of the 95 Tertiary crude oils, only 18% of those produced from formations of less than 3000 feet had -4 P.I. gravities higher than 30" In the production depth range, 3000 to 6000 feet, 54y0 of the oils had
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 11
PETROLEUM-ORIGIN gravities of higher than 30' A.P.I., and in the crudes produced from formations of depths greater than 6000 feet, 97% had gravities higher than 30" A.P.I. I t is evident from these data that producing depth, which reflects increased temperature, is important in relation t o the A.P.I. gravities of crude oils. I n other words, the crude oils found in the formations a t greater depth for the time interval represented by the Tertiary era are almost always high gravity oils, as would be expected if this evolution
TOTAL TERTIARY
GULF COAST TERTIARY
95 CRUDES
67 CRUDES
CALIFORNIA TERTIARY 20 CRUDES
Figure 2 were a result of low temperature cracking to form components of higher A.P.I. gravity than those originally present, I n the other sections of Figure 2 a breakdown between the Gulf Coast and California Tertiary crude oils is given, largely to show that the same trend toward the formation of lighter oils a t lower producing depth is characteristic of both groups of crudes. These groups, of course, represent original environments of deposition separated by a considerable distance geographically. The data may be summarized as follows: Crudes with A.P.I. Gravity > 30", % Total Gulf Coast California Tertiary Tertiary Tertiary (95 crudes) (67 crudes) (28 crudes) 37 0 18 53 54 56 100 90 97
Producing Depth, Feet < 3,000 3000-6000 > 6,000
The distribution of low molecular weight fractions in crude oils produced from rocks of the different major geologic intervals is shown in Figure 3. The percentages of crudes having more than 20% ( a figure taken arbitrarily from a preliminary review of the data) and those having less than 20% of hydrocarbons boiling up to 392' F.-that is, gasoline boiling range hydrocarbons-are shown as a function of geologic era of the sedimentary rocks. Whereas the ratio of crudes having more than 20% of gasoline boiling range components to those having less than 20 % is only about 1.1 in oils of Tertiary age, this same ratio is 11.5:1 in the older Paleozoic crudes. The data are summarized as follows:
> 20%/392 F. < 2OojO/392 F. O O
Low Molecular Weight Fractions in Crude Oils, % of Crudes Tertiary Mesozoic Paleozoic (112 crudes) (33 crudes) (106 crudes) 52 65 92 48 35 8
Figure 4 illustrates the relationship of both age and depth t o the presence of low molecular weight hydrocarbons in crude oils. Data are plotted for the Gulf Coast Tertiary crude oils as a function both of the geologic periods, Pliocene, Miocene, Oligocene, November 1952
and Eocene, and of producing depth. Of all the oils produced at shallow depth (less than 3000 feet) none of the crudes had as much as 20% of gasoline components boiling up to 392' F. I n other words, over the fairly short geologic time interval represented here, none of the shallow crudes had been exposed to a sufficiently high temperature to give oils containing a high proportion of low molecular weight components. It is evident from this figure also that, a t producing depths in the range 3000 to 6000 feet and above 6000 feet, the age of the oil becomes important, and with increased age and depth of burial a progressive brend toward the formation of lower molecular weight hydrocarbons is apparent. The foregoing data indicate that there is considerable evidence of a progressive evolution in crude oils, from the heavy cyclic crudes first formed and found in young sedimentary rocks to the lighter, more paraffinic crudes containing larger proportions of low molecular weight components and commonly found in producing horizons of relatively great depth or age. Table I1 compares the characteristics of a reasonably typical young (Pliocene) Gulf Coast crude oil produced from a shallow depth, with data on aGulf Coast crude of moderately young age (Eocene) produced from a considerable depth. D a t a are also inrluded on two older (Paleozoic) crudes from Devonian and Ordovician sedimentary rocks. The shallow Pliocene formation gave a low A.P.I. gravity crude containing only a small amount of gasoline boiling range components. An appreciable quantity of high molecular weight residuum (boiling above 572" F. at 40 mm.) was present. The Eocene crude produced from 8550 feet, however, showed considerable evidence of evolution in having about the same A.P.I. gravity as the Paleozoic oils and being about the same as the latter in high content of gasoline, low carbon residue, and low content of residuum. I t is concluded that the Eocene oil, although of only moderate age, was transformed to a mature crude resembling the Pennsylvania and Oklahoma crudes as a result of the rather great depth of burial.
TERTIARY
I12 C R U D E S
MESOZOIC 33 CRUDES
PALEOZOIC 106 CRUDES
100-
Figure 3
The present discussion is not intended to show that the nature and composition of all crude oils are due only to chemical changes which occur after the primary crude oils accumulate in reservoirs, but only to emphasize that such changes in crude oils are probable and that there is evidence that they do take place and have an effect on the characteristics of the crudes. The probable influence of differences in the nature of source materials and of environments of deposition on the properties of crude oils ultimately obtained is not excluded. These factors undoubtedly are of general significance and in certain cases may be of paramount importance in determining the character of the crudes obtained. It would be difficult to visualize that the differences between the Pliocene and Eocene crudes (Table 11) and t h e marked
INDUSTRIAL AND ENGINEERING
CHEMISTRY
2559
Table 11.
Characteristics of Crude Oils-Bureau Routine Method
of Mines
Crude. Location
Anse La Butte, Eola, Bradford, Pauls Valley, Louisiana Louisiana PennOklahoma sylvaqia Age Pliocene Eocene Devonian Ordovician Producing depth, ft. 1440-1460 8650 .. 4028-4066 Gravity 'A.P.1. 22.8 44.1 43.6 41.9 Sulfur kt. % 0.17
